Document thread safety

[breyed]

Describe the issue

For the .NET AWSSDK.SimpleEmailV2 SDK, I'm trying to determine whether I can use an instance of AmazonSimpleEmailServiceV2Client from multiple threads simultaneously. Based on #1974, it seems likely, but that issue was for V1.

In general, any classes that are thread safe should indicate so in their docs, so developers have clear answers without having to root through GitHub issues.

Links

https://docs.aws.amazon.com/sdkfornet/v3/apidocs/items/SimpleEmailV2/TSimpleEmailServiceV2Client.html

[normj]

Agree on the doc suggestion but yes all of the SDK service clients are thread safe. Given the cost of creating a service client the best practice is to reuse a client as much as possible.

[dscpinheiro]

I just merged a PR (#4402) updating the service client docs to mention thread safety, this will be included in the next V4 daily release.